Circuit comprisng a combined signal and environmental light sensor

ABSTRACT

In indication and image reproduction apparatus, the ambient brightness must be taken into account for an optimum reproduction quality. This requires an optical sensor, which is associated with additional technical outlay. The invention relates to a circuit which allows the ambient brightness to be determined by means of a sensor that is already present in the device, for the reception of data or control signals.  
     In particular for apparatus appertaining to consumer electronics.

[0001] The invention relates to a circuit for evaluating opticalsignals. It is based in particular on a circuit in accordance with thepreamble of claim 1.

[0002] In devices with luminous indications, displays, or means forprojection, the readability of the indication or the representationquality of images can vary considerably with the brightness of theambient light. Under changing light conditions, the user generally hasthe opportunity to adapt specific operating parameters of the device,such as brightness or contrast, for instance, to the light conditions inthe vicinity of the apparatus. In order to spare the user manualadaptation, various apparatus already have automatic adaptation ofspecific operating parameters to the brightness of the ambient light.The detection of the ambient brightness that is required for automaticadaptation is usually based on an optical sensor fitted in the housingof the apparatus, generally pointing in the direction of the observer.The signal made available by the ambient light sensor is fed to aregulating arrangement which automatically performs optimum adaptationof the operating parameters to the ambient brightness. The applicationof such automatic adaptation is appropriate, inter alia, in televisions,video projection devices, dashboard illuminations or devices with liquidcrystal displays such as portable computers or mobile telephones.

[0003] Although the above-described method for adapting the operatingparameters of an indication or projection apparatus to the ambientbrightness is already used in commercially available apparatus, at thepresent time it is employed only in a small number of, usuallyhigh-quality, devices. The reason for this is that the sensor requiredfor measuring the ambient brightness causes relatively high additionalproduct costs, on the one hand because of the costs for the componentper se but on the other hand because of the precautions which arenecessary for incorporating the sensor in the apparatus housing. Inaddition, the optical sensor can entail restrictions for theconfiguration of the apparatus housing which, under certaincircumstances, cannot be harmonized with a miniaturization or a specificaesthetic configuration of the housing.

[0004] It is desirable because of this to provide a circuit fordetecting the ambient brightness with the least possible additionaltechnical outlay.

[0005] The invention described in claim 1 proposes such a circuit.According to the invention, the sensor that is present anyway in manyapparatus, for the reception of control or data signals, is in this casesimultaneously used for determining the ambient brightness.

[0006] Said sensor is generally fitted in the direction of the user andthus has an optimum position for determining the ambient brightness.Moreover, the spectral range in which (infrared) sensors that arecurrently used are sensitive for control or data signals also covers thevisible part of the spectral range. Since the infrared componentcontained in the ambient light is proportional to the intensity of thevisible light, it is ensured that the ambient light can be detected justas well as by a separate sensor. The separation of ambient brightnessand control or data signals becomes technically possible by virtue ofthe fact that the ambient light is essentially constant and thus bringsabout a signal that varies only slowly at the optical sensor, whilecontrol and data signals are characterized by fast, pulsed changeswhich, for example in the case of the infrared receiver LTM-9237-33 fromthe company Liton, are of the order of magnitude of 30 kHz. Theseparation of control or data signals and ambient brightness can thus beattributed to a separation of fast and slowly varying signal components.

[0007] The simultaneous use of the optical sensor for control or datasignals as sensor for the ambient brightness makes it possible to omitan additional optical sensor, as is currently necessary in regulatingarrangements, and the associated measures such as cut-outs on thehousing, cable feeds, etc. The detection of the ambient brightness indevices which already have an optical sensor for receiving control ordata signals anyway, can thus be implemented significantly morefavourably by means of the invention.

[0008] Exemplary embodiments of the circuit according to the inventionare illustrated diagrammatically in the drawing.

[0009] In the FIGS:

[0010]FIG. 1 shows a first exemplary embodiment of the invention,

[0011]FIG. 2 shows a second exemplary embodiment of the invention and

[0012]FIG. 3 shows a third exemplary embodiment of the invention.

[0013]FIG. 1 illustrates one possible embodiment of a drive circuitaccording to the invention using the example of an infrared (IR) sensorfor the reception of control signals of an infrared remote control. Thecircuit diagram diagrammatically shows an IR diode 10, which is operatedvia a resistor 11 from a voltage source 12 with the voltage U. Thevoltage signal dropped across the IR diode is forwarded on the one handvia a low-pass filter 13, 14 to an output 17 for the regulation of theoperating parameters such as brightness or contrast, for example, and onthe other hand via a capacitor 15 and operational amplifier 16 connecteddownstream to the output 18 to the signal decoder for processing theremote control signals. Thus, in the existing circuit, the rapidlyvarying signal components, which can pass through the capacitor 15upstream of the operational amplifier but are blocked by the low-passfilter 13, 14 are separated from the essentially constant signalcomponents, which, conversely, are blocked by the capacitor and allowedto pass by the low-pass filter. The latter are proportional to theambient brightness and can be fed to a regulating arrangement whichadapts the contrast of the image, for instance, in response to thebrightness of the ambient light. The circuit described disregards thefact that, in the case of high ambient brightness, the voltage acrossthe IR diode 10 can collapse under certain circumstances.

[0014]FIG. 2 diagrammatically shows a further exemplary embodiment of adrive circuit according to the invention, which avoids this problem. Twotransistors 21, 22 are additionally provided, whose bases are connectedto the output of the operational amplifier 26 via a low-pass filter 23,24. The transistor 21 readjusts the current through the infraredreception diode 20 in accordance with the ambient brightness in such away that the voltage across the diode remains virtually unchanged and acollapse in the voltage at high ambient brightness is preclude. Thetransistor 22, whose base is connected in parallel with the base of thefirst transistor, correspondingly supplies a signal which reflects theambient brightness.

[0015]FIG. 3 shows a further advantageous embodiment of the invention,which is based on the combination of a current mirror comprising thetransistors 31, 32 with commercially available IR reception modules 30.The circuit is suitable for reception modules whose current consumptionis essentially determined by the current consumption of the receptiondiode and thus by the ambient brightness. In order to obtain the ambientbrightness, the current taken up by the reception module in this caseneed merely be copied by a current mirror circuit 31, 32 and be filteredby means of a low-pass filter 33, 34. The ambient brightness can then beobtained at the output 17 from the change in the offset value relativeto the basic current consumption in darkness. The circuit thus has theadvantage that commercially available IR reception modules (e.g.LTM-9237-33 from Liton), as are currently used as standard in the massproduction of consumer electronics, can be used unchanged.

[0016] The last-mentioned embodiment, in particular, shows that, byvirtue of the invention, it is possible to realize a detection with anextremely low additional technical outlay.

[0017] It shall additionally be noted that the invention can betransferred without difficulty to other areas by a person skilled in theart. Only a few further examples shall be mentioned at this point, suchas mobile telephones with an IR interface, in the case of which thedisplay brightness is to be regulated in response to the ambientbrightness, IR receivers in motor vehicles, for opening and closing thevehicle, in the case of which the illumination of the interior space orof the indication elements is to be regulated simultaneously, or a slideor video projector whose lamp power is to be regulated in response tothe ambient light.

Environment-Dependent Automatic Luminance Control for Display Screens

[0018] The present invention relates to a method as well as a device,with the aid of which the luminance or brightness, contrast orbackground illumination of display screens is automatically andcontinuously adjusted as a function of the particular average ambientluminance in the field of vision of the user.

[0019] Recent ergonomic studies have demonstrated that the luminancedistribution in the field of vision of the user must not exceed specificproportions in order to be able to work free of fatigue with displayscreens. Thus the luminance studies in the center field of the field ofvision must not exceed a ratio of three to one. In the environment ofthe field of vision luminance differences in a ratio of one to ten arepermissible. Since in offices the luminance distribution varies stronglyas a function of the outside light intensity and weather conditions, themanual setting capability of the display screen brightness and of thecontrast is an unsatisfactory static solution. The brightness of thescreen is only rarely corrected, and if such takes place then onlyinstinctively and guided by perception since there exist no clear andunambiguous reference values. The human eye perceives luminancedifferences only in logarithmic resolution and the adaptation to theparticular luminance of the observed object (adaptation) occurs largelyon the unconscious level. In the absence of sufficiently distinctwarnings, a person will persist in an inadequate luminance environmentuntil the overworked adaptation mechanism in the eye leads to noticeablefatigue. Due to the absence of an understanding of the physiologicalconnections, the cause of the fatigue is still not recognized. Thereforethe person involved can also not take suitable measures (namely theadaptation of the luminances of observed object and environment until anacceptable relationship between the two values is attained).

[0020] The present invention therefore addresses the problem ofproposing a method or a device, with the aid of which the luminance ofthe screen is continuously and automatically adapted to the averageluminance in the field of vision of the user.

[0021] The problem is solved according to the invention thereby that thedisplay screen is coupled with an external light sensor which can beoptionally placed in the room, which monitors the luminance distributionin the field of vision of the user.

[0022] In the following the invention will be described in conjunctionwith FIGS. 1 to 5.

[0023]FIG. 1 depicts the structure of the device. It comprises at leastone light sensor 1, an operational connection 2, a control of thedisplay screen luminance 3, a display screen 4 and a housing 5.

[0024] The sensor 1 can be individually placed in the room and can beadjusted such that the acquired solid angle corresponds approximately tothe field of vision of the user.

[0025]FIG. 2 shows one possible placement of the sensor 1. An electroniccontrol 3 installed in the screen housing 5 or connectable to it by aninterface ensures that brightness and contrast or background lighting onscreen 4 are adjusted such that the display screen luminance is at anoptimum ratio to the average luminance in the field of vision.

[0026] The adjustment buttons for brightness and contrast known onconventional screens could serve the user for the purpose of being ableto match the automatic control of the screen luminance to his ownpersonal preferences. A further capability for matching the controlconsists in the suitable placement and adjustment of sensor 1. Thelatter should in this case fairly accurately cover the solid angle whichmost often is frequented by the particular user during his viewing.

[0027] In the case of current display screens the luminance of the imagearea can be adjusted to between 0 and approximately 100 cd/m². Withthese values the luminance ratio of 1 to 3 in the field of vision canreadily be attained. Maximum adjustment of the screen luminance (100cd/m²) would still ensure an acceptable luminance environment withaverage field of vision luminances of up to 300 cd/m². This value ishardly exceeded in rooms with well functioning sun and antidazzlemeasures.

[0028] One possible control characteristic is shown in FIG. 3. Theaverage luminance in the field of vision L_(G) relative to the screenluminance L_(B) is linear in the lower range. In the limit range thecurve approximates asymptotically the maximum screen luminance. But assoon as the optimum luminance ratio of 1 to 3 is exceeded, a controllight on the screen housing 5 indicates such.

[0029] As an implementation opposite to the external light sensor 1 theintegration of the light sensor 1 into the screen housing 5 itself wouldalso be conceivable. It is also important in this case that the sensor 1(or several sensors) covers the field of vision of the user and that thecovered solid angle can as much as possible be flexibly adjusted.

[0030] One possible disposition of two sensors 1 on the screen housing 5is shown in FIG. 4. The housing 5 here gets two “ears” at the two uppercorners. At each of the tips of the ears is placed one light sensor. Thesensors are adjusted such that the solid angle covered by the twosensors covers the full background of the display screen and correspondsapproximately to the field of vision of the user.

[0031]FIG. 5 depicts one possible solution for the shielding of thesensor 1. The light sensor 1 is enveloped in a displaceable sleeve 6.Depending on the position of the sleeve a smaller or larger solid anglecan be set.

[0032] Consequently a method for the automatic control of the luminanceof display screens is proposed, in which through an externally disposedlight sensor 1 the average luminance in the field of vision of the useris acquired and in which this uniquely quantifiable value serves as areference value for an ergonomically faultless automatic control of thedisplay screen luminance.

[0033] Also proposed is a device for carrying out the method with adisplay screen 4 with housing 5 provided with a control 3 for changingthe screen luminance, in particular the brightness, the contrast and thebackground lighting, and the device comprises at least one light sensor1, which is operationally connected to the control 3 such that bydisposing the light sensor 1 in the direction of viewing, and theshielding of the same onto the field of vision area of the user, theluminance of the display screen 4 is automatically controllable as afunction of the luminance distribution in the field of vision of theuser.

[0034] One possible embodiment of the device is characterized therebythat the at least one light sensor 1 is disposed separately from thescreen 4 at a favorable location in the room, and is operationallyconnected to the control 3 for example by wire or infrared, radio orultrasound transmission.

[0035] The at least one light sensor 1 can also be disposed on thescreen housing 5 itself. In this case the contrast and/or the brightnessand/or the background lighting is also automatically controllablethrough the light sensor 1.

[0036] The device can also be employed with television screens or videoterminals or other data viewing apparatus. It is also possible tocontrol correspondingly a display apparatus, for example for measuringinstruments and clocks.

[0037] The device can also be realized such that through an externallydisposed control the looped-through RGB, video or YC signals areaffected.

[0038] It is recommend to provide the control 3 with at least onecontrol light, with the aid of which an unfavorable luminance ratio ofscreen 4 and environment can be indicated.

1. Circuit for evaluating optical signals with an optical sensor (10),which converts optical signals into electrical signals, the opticalsensor being electrically connected to a drive circuit, the drivecircuit having an output (18), which makes available the data or commandsignals received from the optical sensor, characterized in that thedrive circuit provides, at a second output (17), a signal whichrepresents the brightness of the light in the vicinity of the sensor. 2.Circuit according to claim 1, characterized in that the circuitcomprises means which generate, at the second output (17), a signalwhich varies with the current consumption of the drive circuit (30) andwhose value depends on the ambient brightness.
 3. Circuit according toclaim 1, characterized in that the circuit comprises means forseparating the input signal into fast and slowly varying, substantiallyconstant signal components, the latter generating the signal at thesecond output (17).
 4. Circuit according to claim 2, characterized inthat the second output (17) is connected to the input of a regulatingarrangement which optimizes the operating parameters of an indication, adisplay, a screen or a projection in response to the ambient brightness.